If there’s a bright center to the Universe, astronomers have found the planet it’s farthest from. Called Kepler-16b, it’s a Saturn-like world which has the distinction of being the first discovered to orbit both Sun-like stars in a binary system.

OK, Star Wars references aside, this is pretty cool. Most of the planets being found around other stars are orbiting single stars. A very few — like a possible planet orbiting Gamma Cephei — orbit one of the stars in a binary system, and some (like NN Serpentis b and c) orbit both stars, but one of them is a dead star like a white dwarf or a neutron star.

Unlike those, Kepler-16 is a binary where both stars, though dinky, are bona-fide stars like the Sun, and the planet orbits both. Actually, how it was found is pretty nifty. The orbiting Kepler observatory is designed to stare at over 100,000 stars and detect the tell-tale drop in light when a planet transits (that is, from our point of view passes directly in front of) its parent star. Kepler has found a lot of planet candidates this way — well over 1200!

Kepler-16 is one (OK, two) of those stars (hence the name), located about 200 light years from Earth. The two stars are eclipsing binaries, meaning that we are viewing them from Earth in the plane of their orbit. Twice every orbital period, one of the stars blocks the light from the other and we see the total light from the system dip a little bit. We know of a lot of eclipsing binaries, and their properties are pretty well understood.

But Kepler-16 is different. After observing the pair for some time, a third dip in the system’s light was seen happening at odd intervals. It was clearly due to a planet, but if this object orbited one star or the other, the transits would happen at regular intervals. The staggered time between dips, though, indicated it was actually in a wide orbit around both stars: as it orbits, it blocks one star’s light, then the other’s, and the timing between those mini-eclipses changes as the two stars orbit each other.

What a mess! I would’ve loved to have seen the look on the face of the astronomer who first graphed the change in brightness of Kepler-16 over time.

We can actually determine a bit about the planet, called Kepler-16b. The amount of light it blocks tells us its size (bigger planets block more light), and it turns out to be roughly the size of Saturn — about 100,000 km (60,000 miles) across, 8 times the diameter of Earth. Also, as it orbits the stars its gravity tugs on them, and that can be detected by taking very careful measurements of the spectrum of the two stars (basically, breaking up the light from the stars into individual wavelengths, like a rainbow with a hundred thousand colors). The mass of the planet can be found that way, and again it looks a lot like Saturn: about 100 times the mass of the Earth.

And while there are two stars involved in heating the planet, their light is pretty feeble. Even at its distance of a little over 100 million kilometers (65 million miles) from the pair — roughly the same distance at which Venus orbits the Sun — Kepler-16b is cold: the temperature at its cloud tops (assuming it’s a gas giant like Saturn) would be at best -70°C (-100°F).

So any visions you have of Luke Skywalker standing in the desert with his leg resting on a rock while he wistfully watches the two suns set in the west may have to wait. Even if the planet has a big moon (which these observations cannot yet detect) conditions there would be a bit chillier than on Tatooine. More like Hoth.

Still, this is a very interesting discovery. These kinds of planets probably exist in large numbers, but they’re hard to detect: the orbits of all three components have to line up just right for us to see them. I’ll note that if the planet’s orbit were just slightly tilted so that it didn’t appear to pass in front of the two stars, its gravity might still reveal its presence as it tugs on the stars. Finding more planets like that may just be a matter of time as more binaries are observed. I wouldn’t have given that sort of thing much of a chance, but Kepler-16b has given me A New Hope.

the temperature at its cloud tops (assuming it’s a gas giant like Saturn) would be at best -70°C (-100°F).

Actually that’s not as cold as you are making it sound. The temperature at the top of the troposphere on Earth where the cloud tops would be is regularity below -60°C. While for a Saturn like planet we wouldn’t be standing anyway, but maybe an Earth sized moon with a little extra CO2 could be comfy.

These kinds of planets probably exist in large numbers, but they’re hard to detect: the orbits of all three components have to line up just right for us to see them. I’ll note that if the planet’s orbit were just slightly tilted so that it didn’t appear to pass in front of the two stars, its gravity might still reveal its presence as it tugs on the stars.

That’s basically what I was thinking as I read your post – just how amazing that all three were at juuuust the right angle for us to see this! I love it!

So, how would you calculate the shape of a stable orbit around a binary system? Would it be a simple ellipse focused on the combined center of gravity for both stars, or would it be more complicated than that?

I’d bet that the dynamical situation here would make it very unstable for a planet to orbit in a different plane that the two stars are in. Also, the dynamics of the star forming process makes it likely that every object in the system is on the same orbit, pretty much like here.

Figured that Kepler would eventually come up with a few of these. Reading the discovery paper, it looks like we were fairly lucky to find Kepler-16(AB) b – eclipses only happen roughly 40% of the time due to orbital evolution. Transits across star B will end in May 2014 and resume about 35 years later, while transits across star A will not occur between 2018 and 2042.

From what I can tell from the paper, the mass was not estimated by the spectroscopic method described by Phil Plait in his post, but by the transit timing/duration variations of the binary induced by the perturbations from the planet. The main effect is the orbital evolution of the binary, the light travel time delay caused by the reflex orbit is only around ~1 second.

When I heard that there was this secret that would be revealed thursday I was expecting they found a TRUE earth twin. Not one of those superearths, not a planet around a red dwarf star, but a real earth twin, with a middle-aged sun-like star (maybe slightly cooler or hotter, but not a red dwarf), not a superearth, but a real earth-earth.

Instead it was this, which was a cool find actually, if planets can have stable orbits around binaries that ups the amount of possible earths, but it was still a LITTLE bit of a letdown, because I was thinking they found the mission goal, hence someone from ILM as well as someone from SETI. Honestly nice find as it is, the secrecy and getting an ILM guy and such kind of made it seem more exciting then it was. (or rather then I personally found it. I mean as I said, cool find… just I thought it was one or more earth twins, and that IS in my view the cooler find.)

Ah well, maybe later, eventually Kepler should find SOME of those with all the stars it’s watching. I do hope that within a year or two we can make an informed guesstimate of how many earths we can expect in this galaxy. I’m particularly hoping for worlds massing about in the 0.9 to 1.2 Earth Mass range, a little further into the green zone (not in the inner edge) orbitting G (or K or F) class stars at least 3 or 4 billion years old, that possess a metallicity equal to or higher then the sun. Because personally I’m rather skeptical about both red dwarfs being good host stars (due to the various less then wonderful properties of red dwarf stars) as well as of superearths being particularly suitable for complex life (if any, they might well be venus-like because they’d have the gravity to hold very dense atmospheres.)

Still, this find DOES go to show we’re actually fairly clueless about how solar systems really form in the end. Because we keep finding those things that upend the current models. That’s interesting. I like how little we really know actually, means there’s still interesting things to discover. But you can’t fault a person for hoping for real earth twins with this Kepler telescope.

I thought that you might post about this Phil. It’s a very exciting result, and I can tell you that the reaction to the announcement here at the conference was general excitement. Funnily enough, that same star wars picture was the first slide in the talk – used as justification for why we want to find these systems 😀

Judging from the talk directly afterwards, there are a few other systems that are candidates for the same kind of thing, as well as several that are circumbinary systems without transits.

@andy – you’re right, the discovery team were fortunate indeed to see this. In fact you can already see the transit depths decreasing in the Kepler data.

Andy, you’re right, the single-lined spectroscopic orbit helped to constrain the masses (only the spectrum of star A was bright enough to see), but most of the info came from the dynamics inherent in the transit timing variations.

Well Saturn – a gas giant of similar mass (95 earth masses) – has Titan as one of it’s moons and Titan is bigger than Mercury so I’d say that’s certainly possible. Titan, of course, is one of thr weirdly earth-like yet very alien places in our solar system as the Huygens lander and Cassini have shown. Titan has an atmosphere, rivers, volcanoes, dunefields and lakes. Can’t see why a similarly large – or better larger – moon of Kepler-16b couldn’t have the same.

@16. M :

When I heard that there was this secret that would be revealed thursday I was expecting they found a TRUE earth twin. … Instead it was this, which was a cool find actually, if planets can have stable orbits around binaries that ups the amount of possible earths, but it was still a LITTLE bit of a letdown, because I was thinking they found the mission goal, hence someone from ILM as well as someone from SETI. Honestly nice find as it is, the secrecy and getting an ILM guy and such kind of made it seem more exciting then it was. (or rather then I personally found it. I mean as I said, cool find… just I thought it was one or more earth twins, and that IS in my view the cooler find.)

Temperature~wise this exoplanet is much coooler! 😉

I see what you mean and sort of agree because that would be the logical guess but it’s better than dubious arsenic eating bugs anyhow! 😉

Or Kepler having its funding cut as one person online speculated.

@17. lordbubonicus Says:

@andy – you’re right, the discovery team were fortunate indeed to see this. In fact you can already see the transit depths decreasing in the Kepler data.

But then think about how they may be missing many others by thesame token! 😉

@17. lordbubonicus : But then think about how they may be missing many other such exoplanetary systems by the same token!

Expanding on this a little – to find such a planet in such an orbit so (relatively) quickly (?) seems to me to indicate there’s potentially an awful lot more of them given that most of them will have to orbit a fair way out from their suns thus needing much more time to find and confirm.

Plus there’s needing them to transit -and the odds of that are generally small too esp. for worlds further out from their stars or so I gather.

Has anyone actually done the maths on how many such eclipsing binary “Tatooine” orbital class world there may be? Anyone know, please?

Hmmm…would it be possible for a planet to have a figure-8 orbit around binary suns?
———
That would be totally boss, but probably not stable. Another interesting possibility, though, might be a planet orbiting one of the Lagrangian points of the binary star system. I can imagine a planet at L1, with no night side – just day and dusk.

Also, I think it’s a little unfair to characterize the entire planet of Tatooine as a “wretched hive of scum and villainy.” That statement referred specifically to Mos Eisley and the greater Mos Eisley suburbs, which is all we saw in the course of the movies and was, admittedly, quite wretched and hivey.

But that’s all we saw of the planet. For all we know, there are probably some very nice, ritzy parts of Tatooine. Or, at least, spaceport towns that are comparatively less scummy and villainous.

Also, I think it’s a little unfair to characterize the entire planet of Tatooine as a “wretched hive of scum and villainy.” That statement referred specifically to Mos Eisley and the greater Mos Eisley suburbs, which is all we saw in the course of the movies and was, admittedly, quite wretched and hivey. But that’s all we saw of the planet. For all we know, there are probably some very nice, ritzy parts of Tatooine. Or, at least, spaceport towns that are comparatively less scummy and villainous.

Well actually we did also see in the movies the Sarlacc pit plus a lot of desert, Luke’s home and the terrain journeyed over by C3PO & R2D2, Jabba’s palace plus Tatooine’s pod-racing circuit, Watto’s shop – then his roadside stall and the late sand people’s camp of the abucting Anakin’s mum tribe.

Can’t say that Tatooine looks like an overly nice place to visit although the desert conditions and twin suns would make for interesting skies! 😉

This also makes me realize that science fiction involving planets in a binary system always seems to depict them as scorching hot. Given the number of red dwarfs in the galaxy, it appears that’d be the exception rather then the rule…

Does anyone know the absolute magnitude of and distance between those stars? Sounds like they must be pretty close together to support a circumbinary at that distance.

Still, how cool would it be if each star had its own planet(s) in addition to Kepler-16b?

I wonder about the nomenclature, too. Aren’t stars in multi-star systems often referred to as [star name]A (or B or C)? Now we’ve got planets around those systems to worry about. And what the heck do we do if each star has planets in addition to a circumbinary planet?
Methinks the astronomical taxonomists had better schedule an emergency meeting and hammer this out, pronto!

What orbits are stable in binary systems? I know that a planet can orbit an individual stars in a binary if its semi-major axis is a fifth of the distance between the stars’ closest approach–that’s how Alpha Centauri A and B could each develop their own planetary systems with rocky worlds in the life zone and all–but what constraints govern planets which orbit the two stars together? How far do they have to orbit? Kepler-16b’ semi-major axis of ~0.7 AU is a bit more than 3.5 times the average distance between AB, and presumably its orbit is stable, but …

The odds are against a Mars-massed moon, I think. Titan might be roughly the same size as Mars but it has something less than half of Mars’ mass, it being an ice-rock mixture. There seems to be a fairly stable ratio of the masses of (non-captured) gas giant moons to their planets in the solar system, the moons amounting to 0.0001-0.0002 the mass of their parents.

What I’ve been wondering, and which I haven’t seen addressed, is this: is it possible for there to be a stable orbit in the Goldilocks zone around both stars, or is that just too close to be stable? In other words, while Kepler-16b isn’t Tatooine, is it possible that there’s a 16c in there somewhere?

Whenever there is a talk of exoplanets….. it is always said that it is too far or too near to support life as WE Know it…is there any possibility that life can exist the way we DONT know…..maybe its not carbon based like ours….does anyone think this if it is the case or even if it is possible…?would love to hear the views of good people on this blog….

You’ll be pleased to hear that there are ongoing projects to find both transiting and non-transiting circumbinary planets with Kepler. So even if such systems aren’t transiting, there’s a good chance that the Kepler team will find them.

How would a planet end up orbiting both stars? Is it a matter of something being created due to accretion, or would it be a far-flung planet/merger which was captured by their gravity?

@AK #43

I believe the ‘Goldilocks’ zone is about it being too hot or too cold: too close to the sun and you’d fry any lifeforms, too far away and it’d be too cold. I think though, the Goldilocks zone is more about looking in the most *probable* places, where you increase your chances of finding alien life. No point wasting energy looking in the least probable places, especially when your funding/time is limited.

is there any possibility that life can exist the way we DONT know…..maybe its not carbon based like ours….does anyone think this if it is the case or even if it is possible…?would love to hear the views of good people on this blog….

The difficulty with life as we don’t know it is exactly that – we don’t know anything about it.

How would we go about recognising life as we don’t know it?

It’s hard enough agreeing on a set of criteria by which we would detect life as we do know it, but we seem to be getting close. Liquid water seems to be a key criterion. On Earth, wherever there is liquid water and some means of obtaining energy from the environment, we find life. But if there are forms of life out there that use supercritical CO2, or liquid ammonia, or whatever, as a solvent for their chemistry, we don’t know anything about it and we have very little chance of finding out in the near future.